a) Field of the Invention
The present invention relates to a projector type headlight using an HID (high intensity discharge) bulb as light source, and more particularly to a projector type headlight designed to prevent a coloring caused by color components of the rays of light emitted from the HID bulb, that is, a colored vision of a white object illuminated with the rays of light from the HID bulb.
b) Prior Art Statement
The projector type headlight is small in size but can provide a high luminous flux density, so it has recently been used more and more widely. One of the latest projector type headlights is known from the disclosure in the Japanese Examined Utility Model Publication (Kokoku) No. 5-14404, for example. FIGS. 1(A) and 1(B) are attached hereto to explain the well-known projector type headlight technology. FIG. 1(A) is a schematic side elevation of a projector type headlight, and FIG. 1(B) shows a light distribution pattern of equiluminous curves defined, on a screen, by the rays of light from the projector type headlight.
As seen in FIG. 1(A), the projector type headlight comprises a reflector (of a spheroidal mirror, for example) 1 having two focuses F1 and F2, and a lamp bulb 2 at the first focus F1. Rays of light emitted from the lamp bulb 2 are reflected and converged by the reflector 1 toward the second focus F2. The rays of light thus converged at the second focus F2 are diffused and incident upon a beam-converging convex lens 3. Passing through the convex lens 3, the rays of light are refracted to be generally parallel to each other and projected forwardly of the headlight. There is provided near the second focus F2 a shade 4 to cut off a part of the rays of light passing by near the second focus F2, for thereby defining a light distribution pattern. The hatched part of the light distribution pattern shown in FIG. 1(B) is a shadow of the shade 4 projected as inverted vertically and horizontally. The edge of the shade shadow is called "cut line". The light distribution pattern shown in FIG. 1(B) is provided for theoretical explanation of the illumination of a road surface by a projector type headlight. If the light distribution pattern has such a characteristic as shown, the projector type headlight is suitably usable as an automobile headlight since the rays of light from the headlight illuminates the road surface in front of a car running on a lane (to the left of the V--V line) while it does not illuminate above the horizontal (H--H line) of the opposite lane without any possibility of dazzling the driver of a car running on that opposite lane (to the fight of the V--V line).
In the conventional projector type headlight shown in FIGS. 1(A) and (1B), a halogen lamp is commonly used as the lamp bulb 2, so the headlight can be designed small and provide a high luminous flux density as described in the foregoing. However, if an HID bulb is used as the lamp bulb 2, the projector type headlight can be designed smaller and provide a larger amount of light. But there is a problem that the rays of light from the HID bulb include color components (the driver will sense colors in the rays of light). FIG. 2 is an axial sectional view of the essential part of an HID bulb, schematically showing the buildup of a discharge arc light. There are defined along the shortest path between a pair of discharge electrodes 6c and 6d a zone a of central discharge arc, a zone b enclosing the zone a and a zone c of outer discharge arcs enclosing the zone b. In addition, a zone d is defined at each of the ends of the electrodes 6c and 6d. These zones differ in color from one another. FIG. 3 shows a distribution of wavelengths in the zones a to d in FIG. 2, in which the horizontal axis shows the wavelength with color scales. As evident from FIG. 3, the zone a of central discharge arc and the zone b defined around the zone a are sensed as white, the zone c of outer discharge arcs is as reddish colors and the zones d at the ends of the discharge electrodes 6c and 6d are as bluish colors. As a result, the light distribution pattern in FIG. 1(B) appears colored.